Like most equipment, you get what you pay for. there are very good lights out there, but they aren't cheap. the rechargable systems use battery packs that fit in your waterbottle holders and are bright enough to do what you want. they come with instructions to mount them, but they will cost you.Do it right.

How long do bicycle lights last?

Battery lights should last a lot longer than 1 hour - duration depending on brand & whether the beam is set on steady or blinking.For instance, I use a planet bike beamer 5 LED light which aparently can last up to 100 hours in flashing mode before needing to change the batteriesCheck out the batteries first. Put some fresh one's in & see how they go. I'm partial to using rechargable batteries, as they can be topped off on a regular basis.If it's not the batteries fault, than return the light.

What's the difference between the 5 LED and 3 LED bike light?

The difference is a little more subtle than 2 LED's. The quality of the manufacture of the light is important. Such as reflector(s) and the type of LED used. I have seen 3 LED light sets that are brighter and throw a better beam than 5 LED lights because of the focal point in the hyperbolic curve of the reflector and/or the magnification of the LED. (LED's have little magnifying lens' built into them)All the above gibberish considered -- try both lights before you buy them. See if they have a good beam. Not just bright, but do they illuminate a large area?